Does use of GPU reduces time of execution of .predict() method in Keras/TF model?
Yes, Keras will use the GPU for inference (predictions) as well as training.

TensorFlow code, and tf.keras models will transparently run on a single GPU with no code changes required.

For more information, you can read the TensorFlow GPU guide, but in effect you shouldn't need to do anything to get inference to run on the GPU.